FIG. 1 is an illustration of a conventional nuclear reactor pressure vessel 10 usable with example embodiments and example methods. Reactor pressure vessel 10 may be, for example, a 100+ MWe commercial light water nuclear reactor conventionally used for electricity generation throughout the world. Reactor pressure vessel 10 is conventionally contained within a containment structure 411 that serves to contain radioactivity. A building surrounding the reactor vessel 10, known as a primary containment 411 with drywell 20, serves to house equipment servicing the vessel such as pumps, drains, control rod drives etc.
As shown in FIG. 1 and as defined herein, at least one instrumentation tube 50 extends into the vessel 10 with core 15 containing nuclear fuel. As existing in conventional nuclear power reactors and as defined herein, instrumentation tubes are enclosed within vessel 10 and open outside of vessel 10, permitting spatial access to positions proximate to core 15 from outside vessel 10 while still being physically separated from innards of the reactor and core by instrumentation tube 50. Instrumentation tubes 50 may be generally cylindrical and widen with height of the vessel 10; however, other instrumentation tube geometries are commonly encountered in the industry.
Conventionally, instrumentation tubes 50 permit neutron detectors to be inserted therein through an opening at a lower end in the drywell 20. These detectors extend up through instrumentation tubes 50 to monitor neutron flux in the core 15 at a desired axial position. Examples of conventional monitor types include wide range detectors (WRNM), source range monitors (SRM), intermediate range monitors (IRM), and/or Local Power Range Monitors (LPRM). Additionally, in Pressurized Water Reactors, where vessel 10 is continuously filled with liquid water, a thermocouple monitor, called a Core Exit Thermocouple (CET) may be inserted into a top of instrumentation tube 50 to monitor outlet temperature of the liquid exiting the reactor for steam generators.
As shown in FIG. 1, vessel 10 may include a downcomer region 30 in an annular space separated from core 15 where fluid moderator and/or coolant may enter from a recirculation loop and flow down through downcomer region 30 to a bottom entry point into core 15. Conventionally, downcomer region 30 is outfitted with one or more liquid sensors that permit measurement of liquid presence at a particular level in downcomer region 30. By measuring a liquid level in downcomer region 30, plant operators may be able to approximate a corresponding fluid level in core 15 because core 15 and downcomer region 30 are in fluid connection. This measurement from liquid levels in downcomer region 30 may be used to appropriately operate reactor 10 and/or respond to transient situations where loss of fluid level in core 15 is a concern.